A fuel cell is an electrochemical device in which electrical energy is generated by chemical reaction without altering the basic components of the fuel cell--that is, the electrodes and the electrolyte. Fuel cells combine hydrogen and oxygen without combustion to form water and to produce direct current electric power. The process can be described as electrolysis in reverse. The fuel cell is unique in that it converts chemical energy continuously into electrical energy without an intermediate conversion to heat energy.
Fuel cells have been pursued as a source of power for transportation because of their high energy efficiency (unmatched by heat engine cycles), their potential for fuel flexibility, and their extremely low emissions. Fuel cells have potential for stationary and vehicular power applications, however, the commercial viability of fuel cells for power generation in stationary and transportation applications depends upon solving a number of manufacturing, cost, and durability problems.
The operation of a fuel cell involves supplying fuel to an anode, where the fuel is ionized to free electrons and hydrogen ions. The freed electrons flow through an external circuit to the cathode. The freed hydrogen ions pass through the electrolyte to the cathode, which is supplied with oxygen. The oxygen at the cathode is ionized by the electrons, which flow into the cathode from the external circuit connected to the anode. The ionized oxygen and hydrogen ions react to form water.
Fuel cells are broadly classified by operating temperature level, type of electrolyte, and type of fuel. Low-temperature fuel cells (less than 150.degree. C./302.degree. F.) require a catalyst in order to increase the rate of reaction to a level that is high enough for practical use. Electrodes for low temperature fuel cells usually are porous and impregnated with the catalyst.
Low temperature fuel cells cannot be used successfully for vehicular propulsion unless the fuel cells have a very large electrode area coated with catalytically active material. The noble metal catalysts used in low temperature fuel cells generally perform most efficiently if they are in small clusters of nanometric dimensions on a suitable support. The support material must be: (a) electrically conducting; (b) chemically inert in the fuel cell environment; (c) mechanically robust; (d) sufficiently adherent to the cell membrane; and, (e) depositable in a thin film form, into which platinum, or other catalyst material, can be incorporated.
A favored material for use as an electrode support material is carbon. The carbon typically is "doped" with 1-10% platinum or platinum-ruthenium. In the past, the catalyst-doped carbon has been applied in the form of ink or using complex chemical processes that require high temperature firing, resulting in a glassy carbon that contains platinum oxide. The high temperature firing that is used to produce these electrodes cannot be used to coat the ionomer membranes that are favored for use in polymer electrolyte fuel cells (PEFC's).
Some of the most efficient electrocatalysts for low temperature fuel cells are noble metals, such as platinum, which are very expensive. Some have estimated that the total cost of such catalysts is approximately 80% of the total cost of manufacturing a low-temperature fuel cell. The expense of such catalysts makes it imperative to reduce the amount, or loading, of catalyst required for the fuel cell. This requires an efficient method for applying the catalyst. The method also must produce an electrocatalytic coating with a minimal catalyst load which also has sufficient catalytic activity for commercial viability.